for some reason this year there has been a black film above the baseboard heater on the walls. Which washes off but is annoying. We have gas forced hot water system. I've never seen this before. It only appears above the baseboards in all the rooms. We recently had the furnance service by Gas company & we got a clean bill of health. We also have a CO detector in the bedroom & nothing is register on that. Could it be due to cold walls vs heater adhering dust.